Before you start, you need the following:
Catchments: Types, area and location of catchments
From the site plan or from actual measurements, find out the different types of catchments that are available for harvesting rainwater, the dimensions of each of these types of catchments and the location.
Types of catchments: From the site plan, mark out the different types of catchments that are available for water harvesting. The type of catchment such as paved or upaved and roof or other paved ground surfaces will determine the collection efficiency and quality.
Area of catchment: The amount of rainfall that you can collect depends directly on the area of the catchment - the larger the surface area, the more the water. Regardless of the shape of the roof, the area for calculation of rainfall collection would be the area equivalent to the area under the roof.
Thus, area of catchment (C) = length (l) x breadth (b)
In the case of catchments at ground level, the area can be found out from the measurement given on the site plan. For irregular dimensions of ground level catchments such as a winding driveway, break it into measurable shapes such as rectangles or triangles and measure.
Rainwater runoff from the following catchments should be avoided:
There are four types of rainfall information that you need:
Geological and hydrogeological data
For systems where the harvested rainwater will be used to recharge the aquifer, selection of site is important. Information must be collected on the following
|Soil||Poor or well sorted sand or gravel, fine sand, silt, loam, layered or unweathered clay|
|Rocks||Fractured or massive rocks, sandstone, limestone|
|Aquifer||Confined or unconfined, perched, thickness of aquifer|
|Depth of water table||Shallow or deep water table zones|
Soil: The soil must have properties that will allow the easily water to move downward. Sand, sandy loam and loamy sand soils have high infiltration rates even when they have high moisture content and low runoff. Infiltration and water movement will be greater in materials, which have greater porosity and permeability such as sands, gravel or fractured rock.
Aquifer: The aquifer must not be at shallow depths and should be at least 8-10 metres below the ground level. The aquifer should be unconfined and must have good hydraulic conductivity as well as transmissivity so that the water that is recharged is quickly spread horizontally to prevent a water mound forming below the surface.
Geology: In rocky areas, you need information about the nature of rocks, whether they are fissured or weathered and their capacity to hold and release water.
|Type of soil/rock||Characteristics|
|Gravel, sand||Porous and permeable|
|Clay lenses interbedded with sand||Partly impervious|
|Clay, shale, mudstone||Porous but practically impervious|
|Basalt, granite and quartzite||Neither porous nor permeable|
The size of the water harvesting is determined by two factors – how much is needed and how much is available.
Objectives and uses
Legislation and incentives
Today many state governments and city municipalities have passed laws that make it mandatory for existing or new buildings to have rainwater harvesting systems. You must find out the details of these laws in order to make sure you are compliant on all counts. At the same time, there are also many incentives to motivate people to take up rainwater harvesting and you must find out about what incentives are available in your city.